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INDUSTRIAL CHEMISTRY. THE PRODUCTION OF NITRIC ACID. Many reactions proceed too slowly under normal conditions of temperature and pressure.

INDUSTRIAL CHEMISTRY THE PRODUCTION OF NITRIC ACID Many reactions proceed too slowly under normal conditions of temperature and pressure. Some reactions proceed at very fast rates but produce very small quantities of product. In order to maximise profits and to reduce costs to consumers, industries aim to minimise the costs of industrial processes. This involves a consideration of yields and rates. The reactions that cause greatest concerns to industries include: 

Reactions with low equilibrium constants. Low equilibrium constants result in low yields of product. For example: The production of ammonia, nitric acid and sulfuric acid.



Reactions with slow reaction rates.



Exothermic processes. Lower temperatures are required to increase yields, however, this results in slower reaction rates. Industries will generally employ lower temperatures and use catalysts to compromise on the decreased reaction rates. For example: The production of sulfuric acid, nitric acid and ammonia.

THE GREATEST COSTS ASSOCIATED WITH INDUSTRIAL PROCESSES INCLUDE: 

The costs of raw materials. To maximise profits, yields are maximised.



Generating high pressures. Industries avoid using extremes of pressure to maximise the yield of product as high pressures require very powerful and expensive pumping equipment together with vessels that can withstand the high pressures. These added costs may not justify the use of higher pressures, and in many cases, it is more profitable to lower the pressure and obtain a lower yield of product.



Generating high temperatures. Industries decrease these costs by using heat evolved in exothermic processes to fuel other reactions in the plant.



The time required to produce the product. Rates are increased by using appropriate catalysts.

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Unit 4 Chemistry – The Production of Nitric Acid

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Note: Industries use processes that use less energy to decrease costs and preserve finite sources. For example: 

The heat produced in one stage of a chemical process is frequently recycled and used to heat other stages of the process.



The heat exchangers which remove and recycle heat operate 24 hours per day so that the enormous costs associated with warming up equipment are avoided.



Industries often exist as integrated complexes i.e. A collection of related industries are located within a close proximity of one another. The by products of one industry (eg. heat) can then be used as a raw material for another industry, reducing wastage, environmental pollution and costs.



If sufficient thermal energy is produced, it may be possible to convert it to electrical energy for use in the plant. In some cases, excess supplies are sold to an electricity supply grid.

MAXIMISING YIELDS Industries will attempt to maximise yields by manipulating Le Chatelier’s Principle. Yields may be cost effectively increased by changing the following reaction conditions: 

Adding an excess amount of the cheaper reactant.



Periodically removing products.



Changing the temperature and pressure of the reaction system.

MAXIMISING RATES As time has a significant impact on the cost of products and staff, industries will also attempt to maximise the speed or time taken to produce a product. Conditions that favour fast reaction rates include: 

High reactant concentrations.



High pressures.



High temperatures.



High surface areas.



Use of catalysts.

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FACTORS INFLUENCING THE METHOD IN WHICH A CHEMICAL IS PRODUCED The operating conditions and any compromises that are required are determined by running small scale experiments, and choosing the set of conditions that will maximise profits i.e. those conditions that result in the highest possible yield of product in the shortest possible time. Other considerations include: 

Raw materials – cost, availability, purity, safety.



Environmental impact – pollution, storage/hazards of waste products, use of water bodies to cool equipment.



Transporting of raw materials and product.



Location of plant.



Availability of necessary technology.



Availability of appropriately qualified staff.

TYPES OF CHEMICAL PROCESSES Batch Processing In this process, fixed amounts of reactants are mixed to produce fixed amounts products. This method is usually reserved for the production of small amounts of product and/or reactions that display high equilibrium constants. Continuous Flow Processing In this process, reactants are continuously supplied at one end, to produce a continual supply of products, which are then removed at the other end of the processing line. This process is only cost effective if sufficient demand exists for the large amounts of products derived via the process. Continuous flow processing also allows for greater control over reaction conditions, making it the preferred technique for many large scale operations. Reactants may be added or products removed at any stage of a process to increase product yields.

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PLANT LOCATION AND STRUCTURE Factors to Consider: 

Accessibility to raw materials.



Transportation costs.



Availability of energy/power sources.



The price of land.



Availability of water supplies.



Storage of raw materials and waste products.



Disposal of waste products.



Pollution and its effects on the environment.



Recycling energy, water and waste products.

GREEN CHEMISTRY Green chemistry involves the design of chemical processes and products that reduce or eliminate the use and generation of hazardous substances in the manufacture and application of the products. By eliminating and reducing waste from chemical processes, green chemistry aims to develop a sustainable approach to a cleaner environment that is beneficial to both our society and the economy. The hazards that green chemistry aims to avoid completely include:    

Toxicity. Physical hazards like explosions. Impact on global climate change. Depletion of resources.

The major difference between green and environmental chemistry is that environmental chemistry focuses on pollution control once the pollutants have been produced whereas green chemistry aims to avoid pollution in the first place.

© The School For Excellence 2012

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THE 12 PRINCIPLES OF GREEN CHEMISTRY Taken From Heinemann Chemistry 2 1.

Prevent waste It is better to design chemical processes to prevent waste than to treat waste or clean it up after it is formed.

2.

Design safer chemicals and products Design chemical products to be fully effective, yet have little or no toxicity.

3.

Design less hazardous chemical syntheses Methods should be designed that use and generate substances with little or no toxicity to humans and the environment.

4.

Use renewable raw materials Use starting materials that are derived from renewable resources such as plant material rather than those such as from fossil fuels that will eventually run out.

5.

Use catalysts, not stoichiometric reagents Minimise waste by using catalysts in small amounts that can carry out a single reaction many times. They are preferable to stoichiometric reagents, which are used in excess and work only once.

6.

Avoid chemical derivatives Avoid using blocking or protecting groups or any temporary modifications if possible. Derivatives use additional reagents and generate waste.

7.

Maximise atom economy Design syntheses so that the final product contains the maximum proportion of the starting materials. There should be few, if any, wasted atoms.

8.

Use safer solvents and reaction conditions Avoid using toxic solvents to dissolve reactants or extract products.

9.

Increase energy efficiency Energy requirements should be minimised. Run chemical reactions at room temperature and pressure whenever possible.

10. Design for degradation Chemical products should be designed to break down to harmless substances after use so that they do not accumulate in the environment.

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11. Analyse in real time to prevent pollution Include continuous monitoring and control during process to minimise or eliminate the formation of by-products. 12. Minimise the potential for accidents Design chemicals and their forms (solid, liquid or gas) to minimise the potential for chemical accidents including explosions, fires and releases to the environment.

BENEFITS OF GREEN CHEMISTRY Some of the many benefits of a green chemistry approach include: 

Higher atom economy.



Advocating energy efficient processes.



Lowers cost of production and regulation.



Less wastes.



Safer products.



Healthier workplaces and communities.



Protects human health (end-users) and the environment.



Offers businesses a competitive advantage in the market place.



Economical stimulus.

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YIELD VERSUS ATOM ECONOMY The yield of a reaction tells us how efficient a reaction is in terms of the amount of product we obtain, relative to the maximum we could get from the amount of reactants we used. It is calculated using the formula: % Yield

=

mass of product obtained (g) x 100 theoretical yield (g)

However, it does not take into account the waste products. Efficient chemical processes have high atom economy, and are important for sustainable development. Atom economy is determined by measuring the amount of starting materials that are incorporated into the desired products, and distinguishing them from those that are wasted (incorporated into undesirable products). Atom economy can be calculated by: % Atom economy =

Relative Molar Mass of Desired Product X 100 Sum of Relative Molar Masses of all Products

A given chemical reaction might have high yield but low atom economy, hence not be seen as a adhering to green chemistry guidelines. WORKED EXAMPLE 1 (a)

Calculate the percentage atom economy of CH 2Cl2 , which is formed according to the following chemical equation: CH 4( g )  2Cl2( g )  CH 2Cl2( aq )  2 HCl( aq ) % Atom economy 

(b)

85  100  53.8% 85  36.6

Would this method of CH 2Cl2 production be considered as a “Green” process? Give a reason for your answer. An atom economy of 53.8% is particularly poor, and this is a very wasteful process. This would not be considered a green process, as one the key principles of green chemistry is that it is better to develop reactions with fewer waste products than to have to clean up the waste (eg. achieve high atom economy).

(c)

How could a chemical company maximise their profits from this chemical process? Use waste products in other chemical reactions. The by-product is hydrogen chloride, which can be sold as a gas or made into hydrochloric acid. These useful substances can then be sold, reducing the potential wastage from the initial process. Alternatively, waste products that are non-toxic and biodegradable are favourable.

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WASTE MANAGEMENT AND POLLUTION IN THE CHEMICAL INDUSTRY A waste product is an unusable or unwanted substance produced during or as a result of a chemical process. Chemical waste is generated in many chemical processes and if not managed correctly, can impose adverse effects on human health and the environment. Responsible industries therefore practise sound waste management by implementing the following actions: 1.

Prevention

2.

Elimination

3.

Reduction

4.

Recycling

5.

Treatment

6.

Disposal

WASTE TREATMENT There are many different forms of waste treatment including: 

Landfill



Dumping at sea



Dispersion in controlled amounts in water or air



Vitrification (sealing in molten slag)



High-temperature incineration ( 1100 C )



Removal of pollutants from waste gases and liquids



Storage in sealed drums in secure locations



High-temperature steam and water treatments.

o

Which treatment process is used by industries depends upon: 

The physical form of the waste



The hazardness of the waste



Threats to animals, people and the environment



The cost of the process

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NITRIC ACID (HNO3) USES OF NITRIC ACID In terms of production, nitric acid is the third most widely produced acid across the world. It has a wide range of uses in agriculture, industry and medicine where it is used as a fertiliser and in the manufacture of fireworks, explosives, medicines, dyes, food preservatives, pesticides and detergents.

PROPERTIES OF NITRIC ACID Nitric acid: 

Is colourless in its pure form but may become orange or reddish in colour if contaminated by nitrogen oxides.



Is highly corrosive.



Is a poisonous liquid (freezing point -42˚C, boiling point 83˚C).



Reacts with water or steam to produce heat and toxic, corrosive and flammable vapours.



Can cause severe burns.



Miscible in water at all concentrations.



Has an acid dissociation constant (pKa) of −1.4. In aqueous solution, it almost completely (93% at 0.1 mol/L).



Will decompose at higher temperatures to form nitrogen oxides.

Nitric acid is both a strong monoprotic acid and a strong oxidant, particularly when hot and concentrated.

© The School For Excellence 2012

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OXIDISING PROPERTIES The products of the reaction between nitric acid and metals depends upon the reactivity of the metal and the concentration of the acid. As a general rule, oxidising reactions occur primarily with the concentrated acid, favouring the formation of nitrogen dioxide (NO2). Reaction between a reactive metal and dilute acid (